In many technological processes, and in particular in those in the food field, products may be subjected to a heat treatment in order to dry, dehydrate, defrost, cook, pasteurize, sterilize or otherwise treat them thermally. The heat to be supplied to the product may be transferred by means of external heat sources by utilizing convection, conduction or radiation effects, or may be generated directly within the product. In the latter case, an oscillating electromagnetic field can be used to produce a field of electrical currents in the product to be treated; by interacting with the material constituting the product, this field brings about a rise in its internal temperature. The electromagnetic field which can be applied may have various intensities and various oscillation frequencies.
Known apparatus which performs heating of this type comprises a radio-frequency generator which, when supplied with the mains voltage, produces an oscillating voltage of variable amplitude and of predetermined frequency at its output terminals. The apparatus further comprises an applicator device with capacitive or inductive behaviour which transforms the oscillating voltage of the generator into electromagnetic fields having predominantly electrical or magnetic components of the oscillating field, respectively. The above-mentioned frequencies are typically within ranges established by international standards, the central values of which are 6.78-13.56-27.12-40.68-433.92 MHz. The intensity of the oscillating electromagnetic fields thus generated depends on the amplitude of the radio-frequency voltage which is applied to the terminals of the applicator device and on the construction of the applicator device, which can increase or reduce the intensity of the field in the zones provided for housing the product to be treated.
There are known devices of various types which apply radio-frequency electromagnetic fields to products having different physical, dimensional and electrical characteristics. The most widespread applications relate to heat treatments of paper, fabrics, textile materials in general, particularly after dyeing, hides, rubber, wood, plastics laminates and food products. In most of these cases, the heating of the products takes place by dielectric losses due to the displacement currents which are induced in them by the electromagnetic field applied, rather than by the conduction currents. Bach of these devices has a structure and technical characteristics suitable for the frequency used and for the type of application required. Moreover, the same devices may have a structure such as to give rise to electromagnetic fields of predetermined intensities, delivering specific powers to the product on the basis of the particular heat treatment required.
For example, the Applicant's European patent application EP 0946104 describes an industrial apparatus for heating food products by means of a radio-frequency oscillating electromagnetic field. This apparatus can cook meat-based food products such as, for example, ham and the like, which have considerable mass and volume and are preferably placed in moulds.
One of the main limitations to the use of radio-frequency technology for heat treatment is that it is difficult to bring the products to be treated to high temperatures within extremely short times without thereby causing undesired effects detrimental to the products. For example, but in non-limiting manner, during pasteurization or sterilization processes, or during the defrosting of purees of food products, it is necessary to supply high powers within a short time and, in particular, high powers per unit volume of product, for continuous flows. This operation is particularly difficult, especially for semiconductive dielectric products, since their considerable electrical conductivity limits the effect of the dielectric losses, that is, the effect due to the displacement currents induced therein by the electromagnetic field applied.
Moreover, if the volume of product transported through the radio-frequency treatment zone per unit of time is particularly high, undesired chemical and organoleptic effects may arise in the products if the radio-frequency treatment time is not sufficiently short.
In some known solutions, it is possible to deliver high specific powers to the products in a short period of time with the use of high voltages and currents in the devices. However, these solutions lead to difficult management and complex control. In fact, the supply of high powers may lead to undesired electrical discharges between various points of the applicator devices, leading to damage thereto and to the product housed therein and, even more disadvantageously, may lead to uneven heating and/or burning of portions of the product.
Processes for pasteurization and/or sterilization of fluid food products should, in theory, comprise a first stage of instantaneous heating of the product to the pasteurization and/or sterilization temperatures, a subsequent stage characterized by a standing time reduced to zero and, finally, a stage of instantaneous cooling to the starting temperature. Since a process of this type cannot be achieved by known technology, the best conditions must be sought in dependence on the apparatus available in order to construct a plant which enables a process as similar as possible to the theoretical one to be achieved.
Thus, in the field of the heat treatment of milk, one of the most widespread pasteurization processes consists of a stage in which the milk is heated to a temperature below its boiling point, a stage in which the product is kept in these conditions for a predetermined period of time, and a cooling stage. The period of time at constant temperature must be long enough to kill pathogenic and sporiferous microorganisms of all types which are present and a proportion of micro-organisms which are not pathogenic but which can nevertheless bring about changes of various types in the product.
Some examples of known heat treatments are slow pasteurization (comprising, amongst other things, a stage of heating to 63°-65° C. and a maintenance stage of about 30 minutes) and quick pasteurization, known as H.T.S.T. (comprising, amongst other things, a stage of heating to 72° C. and a maintenance stage of about 20 seconds). Both processes serve to pasteurize milk intended for consumption or for the production of dairy products such as cheese, cream, butter, curd cheese and the like.
Another known type of method for the heat treatment of milk is that used when the product is intended for direct consumption. In this process, which is known as U.H.T. the milk is heated to temperatures much higher than the pasteurization temperature, which are maintained for a period of time much shorter than in the processes described above, for example, 155° C. for about 2-5 seconds. In this case, the object is to eliminate as far as possible everything which leads to a reduction in the shelf life of the product since, in this case, the milk must have a shelf life of at least 120 days when kept at ambient temperature. In U.H.T. treatment, the heating stage is usually achieved in two steps: first of all by an indirect exchange, for example, by means of external heat sources, such as plate-like hot-water heat-exchangers, and then by a direct exchange, for example, by the admission of steam into the milk at high temperature (known as the “uperization” stage)
Both of the treatments described above have some disadvantages. The use of steam at high temperature enables the product to be brought to the required temperatures quickly but at the same time leads to an alteration in its physical and organoleptic characteristics. In fact, the steam admitted not only changes the percentage of water in the milk, but also leads to depletion of the nutritional substances since, once the required temperature has been reached, the same amount of steam which was injected is extracted, together with some substances which were originally contained in the milk. It is therefore an invasive technique in which an external element, the steam, is used to achieve extremely quick heating times which cannot be achieved by other known techniques. In contrast, indirect exchange treatment does not modify the chemical characteristics of the product, since it is not an invasive process, but is not as effective in reaching the required temperatures within a short time, thus causing serious and undesired side effects in the pasteurized milk.
In the said processes, it is also known that, for a given maximum treatment temperature, the milk-sterilization effect can also be achieved with a limited duration of the maintenance stage, which is responsible for the organoleptic degradation of the product, if the required temperatures are reached within the shortest possible time. It is clear that, with known techniques, there are physical limitations to this which are connected with the technologies used. For example, indirect exchange of heat by means of heat exchangers requires sufficiently long periods of contact between the product and the heating means for the product to be heated completely and uniformly in order to ensure complete treatment thereof.
It is known that, over the years, the temperatures necessary to achieve correct sterilization of milk are continually increasing because of new contaminations connected with new pathogenic agents, spores, enzymes, bacteria, or microorganisms. These harmful substances are in fact becoming ever more heat-resistant and hence difficult to inactivate. However, excessive heat treatment clearly conflicts with the ever greater requirement by the public for products with flavours, odours and colours which are as close as possible to their natural properties. These reasons have forced and are forcing many companies producing sterilization plant or fluid foods to investigate, implement and use ever newer and more sophisticated technological processes connected with the improvement of plant, with the use of heat sources which make use of convection, conduction or radiation effects. In this connection, it is widely believed that the known technologies have reached such a high degree of development that they can now be considered “mature techniques” which are ever more difficult to improve.
The main object of the present invention is to overcome the disadvantages of known apparatus by providing an industrial apparatus for applying radio-frequency electromagnetic fields to semiconductive dielectric materials which can treat large quantities of product very quickly without bringing about particular changes in its physical, chemical and organoleptic characteristics.
Another object of the present invention is to provide an industrial apparatus for the heat treatment of semiconductive dielectric materials which can apply radio-frequency electromagnetic fields of considerable intensity with the use of limited voltages and currents in the apparatus so as to be easy to manage and control, avoiding unnecessary loadless power dissipation and localized heating, and consequently burning, of the product.
A further object of the present invention is to provide an industrial apparatus for applying radio-frequency electromagnetic fields to semiconductive dielectric materials which can easily apply fields of greater or lesser intensity for times adjustable over an extremely wide range and which can thus be used in various technological processes which may even involve products other than food products.
Another object of the present invention is to provide an industrial apparatus which is easy and inexpensive to manufacture, easy to inspect, easily accessible for cleaning and maintenance operations, and easy to dismantle.
To achieve the objects indicated above, the subject of the invention is an apparatus having the characteristics indicated in the appended claims.
According to a particularly advantageous characteristic of the present invention, the industrial apparatus comprises means for housing and transporting the products to be treated, which means can be connected easily and quickly to the means used for transporting the products in production lines. The apparatus of the present invention can thus be inserted in production lines for fluid products, preferably of low viscosity, transported in sterile pipes, without altering the layout and configuration of the existing devices or even the cross-section and/or configuration of the lines and pipes for transporting the products.
Another advantage of the present invention is that products flowing through production lines at extremely fast speeds can be treated industrially without the provision of additional applicator devices and/or plants. As well as having large dimensions and therefore being difficult to use and to maintain, these additional devices generally require large volumes of product in the radio-frequency treatment zones and therefore involve treatment times which are so long that they cause undesired chemical/organoleptic effects in the products.